The invention relates to a signal system.
In a signal system which deals with voltage signals of a very high and a very low magnitude, it may be sometimes desirable to accept both levels of signals while it may be desirable at other times not to accept any one of them. There is no problem in providing an arrangement to accept both signals. However, in view of the considerations of circuit components, it may be difficult to prevent both signals from being accepted. A signal of a low magnitude can be blocked, while a signal of much greater magnitude cannot be blocked.
Such difficulty may be illustrated by an auto strobe unit used in combination with a photographic camera. Referring to FIG. 1, there is shown a camera AC including a taking lens 1, and the light passing therethrough impinges upon and is reflected by either a shutter blind 2 or film surface 3 to be measured by a light receiving element 4. The output of the element 4 is utilized to control a shutter control system 5. When the control system 5 turns transistor 7 off, a back electromotive force is developed across an electromagnet 6, which is usually used to hold a second shutter blind. The back e.m.f. is fed to the control of an auto strobo unit AS from a pair of terminals A.sub.1, B.sub.1 in order to interrupt the flash illumination thereof. A strobo unit constructed in this manner will be referred to hereinafter as TTL auto strobo unit, which is known in the art.
The TTL auto strobo unit includes a circuit arrangement as shown in FIG. 4. Referring to this Figure, a discharge tube 20 is connected across a power source E through a flashlight circuit 21 of a known form including a trigger transformer T.sub.1, diode D.sub.0 and resistor R.sub.0, the arrangement being such that a flashlight illumination is initiated when synchro contacts 22 are closed. A thyristor 23 is connected in series with the discharge tube 20 in order to interrupt the flashlight illumination from the discharge tube 20 when an extinction discharge tube 24 conducts to apply a back bias thereto through a commutating capacitor 25. The discharge tube 24 is connected with a transformer T.sub.2, to which is also connected a closed loop circuit including a trigger capacitor 26 and another thyristor 27. The capacitor 26 is initially charged from the source E through a path including resistors R.sub.7, R.sub.6, diode 28 and a common line Cm, to which is connected a terminal B.sub.1 which is adapted to be connected with a terminal B.sub.0 provided on the part of the camera. The gate of the thyristor 27 is connected through diode 29 and resistor R.sub.5 with a terminal A.sub.1 which is adapted to be connected with an output terminal A.sub.0 provided on the part of the camera, and the gate of thyristor 27 is also connected through resistor R.sub.2 with the common line Cm. The terminal A.sub.1 can be selectively connected with the output terminal A.sub.0 of the camera or with the output terminal D.sub.0 of an external photometric unit PD (see FIG. 1) which is associated with the auto strobo unit AS. The cathode of the thyristor 27 is connected with the common line Cm through diode 28, and is also connected through resistor R.sub.3 with the emitter of transistor 30, the base of which is connected through diode 31 and resistor R.sub.5 with the terminal A.sub.1. A parallel combination of a Zener diode 32 and capacitor 33 is connected between the collector of transistor 30 and the common line Cm to provide a constant voltage source for the components such as transistor 30 when the charge is transferred from the capacitor 34 thereto as the synchro contacts 22 are closed. The capacitor 34 has been initially charged as well as the capacitor 26. It will be noted that the terminals A.sub.1, B.sub.1, resistors R.sub.2, R.sub.3, R.sub.4 and R.sub.5, thyristor 27, transistor 30 and diode 31 collectively form a signal system ST for interrupting the flashlight illumination from the strobo unit.
Referring to FIGS. 2(A), (B) and (C), there are shown the voltage applied to the auto strobo unit AS, indicated on the ordinate, plotted against the shutter operating time shown on the abscissa during several typical modes of operation. Specifically, FIG. 2(A) illustrates a normal strobo photographing operation, FIG. 2(B) a shutter operation with an exposure period on the order of 1/60 second which is close to a strobo photographing operation, and FIG. 2(C) a shutter operation with an exposure time less than 1/60 second, which is shown as a typical shutter period of 1/125 second.
Considering the shutter operation illustrated in FIG. 2(A) initially, a shutter release takes place at time t.sub.0, and a power switch (not shown) provided on the part of the camera is closed simultaneously. During the initial phase of closure, the power switch experiences mechanical oscillations, which cause a chattering of the switch contacts. As a consequence, the electromagnet 6 is repeatedly turned on and off, producing a sawtooth-shaped back e.m.f. signal W.sub.0 having a magnitude on the order of -60 V. When the power switch assumes a stable on position, the electromagnet 6 remains in a stabilized energized condition, and the element 4 initiates its photometric operation under this condition. At this time, a first shutter blind 9 will have passed across a film surface 8 to maintain it exposed, as shown in FIG. 3 while a second shutter blind 10 is held in its starting position by means of the electromagnet 6. In FIG. 3, the terminal position at which the first blind 9 remains stationary is indicated by position B'.sub.1 of its trailing edge 9a while the starting position of the second blind 10 is indicated by position B.sub.2 of its leading edge 10a. When the shutter blinds 9, 10 assume the positions B'.sub.1, B.sub.2, respectively, the synchro contacts 22 (see FIG. 4) are closed and hence a flashlight illumination is produced by the discharge tube 20 at time X as shown in FIG. 2(A), the profile of the flashlight illumination being indicated by a curve W.sub.1.
When the element 4 determines that a proper exposure is reached, the shutter control system 5 produces a signal to deenergize the electromagnet 6, whereupon the second blind 10 is freed to run. The second blind 10 begins to run from the starting position B.sub.2 and reaches a stable position after running through an initial distance e. It begins to cover the film surface 8 at a position B'.sub.2, and continues to run to cover the film surface 8 completely, and finally stops while maintaining the film surface 8 covered. Normally, the synchro contacts 22 are designed to be closed when the first blind 9 reaches the position B'.sub.1 and when the second blind 10 is located between the positions B.sub.2 and B'.sub.2.
Assuming that the electromagnet 6 is deenergized at time 1/Z, a strobo interrupt signal W.sub.2 shown in FIG. 2(A) is produced by the back e.m.f. to the terminal A.sub.1 as the electromagnet 6 is deenergized, thus interrupting the flashlight illumination of the strobo unit. It will thus be seen that the strobo interrupt signal is produced after the synchro contacts 22 are closed to apply a circuit voltage to the transistor 30. Hence, the interrupt signal renders the transistor 30 conductive, whereby thyristor 27 is fired to cause the conduction of the extinction discharge tube 24, thereby interrupting the flashlight illumination. After the strobo interrupt signal is produced, a signal L.sub.0 of a low voltage on the order of -2.5 V is applied to the terminal A.sub.1 of the strobo unit AS from a power source contained in the camera during the time the power switch remains closed after the electromagnet 6 is deenergized, as shown in FIG. 2(A).
Referring to FIG. 2(B) to describe a shutter operation which takes place with an exposure period of 1/60 second, very close to the time X utilized during a strobo photographing operation, the element 4 will indicate a proper exposure 1/60 second after the initiation of running of the first blind 9, thereby causing the shutter control system 5 to produce a deenergize signal to be applied to the electromagnet 6, followed by a strobo interrupt signal W.sub.3. At the time of 1/60 second after t.sub.0 when the electromagnet deenergize signal and the strobo interrupt signal are produced, the first blind 9 generally still continues to run and the trailing edge 9a will be running at position B.sub.1 as shown in FIG. 3. The first blind 9 will reach the position B'.sub.1 when the second blind 10 is freed for running at its initial position B.sub.2 to run until its leading edge 10a reaches the position B'.sub.2. At this time, namely, at time X' second, the film surface 8 is fully exposed, and the synchro contacts 22 are closed to provide a flashlight illumination as indicated by a curve W'.sub.1 during the time the first shutter blind 9a is at position B'.sub.1 and the second shutter blind 10a is between positions B.sub.2 and B'.sub.2. As to the detailed construction of such X-contact operation, it may be seen in U.S. Pat. No. 3,987,468, for example. This flashlight illumination is produced independently from the fact that a proper exposure as determined by the element 4 is already reached, and is therefore unnecessary. Since the strobo interrupt signal W.sub.3 has been already produced, there is no longer means available which can be utilized to block the flashlight illumination, resulting in a full cycle of the flashlight illumination. Since this illumination occurs during the time the film surface 8 is entirely exposed, the unfavorable result will be evident. Thus it will be seen that there must be provided some means other than the strobo interrupt signal W.sub.3 which can be utilized to interrupt the flashlight illumination during the time the synchro contacts 22 are closed.
It will be most desirable to utilize a low voltage signal L.sub.0 which is supplied from the camera to the terminal A.sub.1 subsequent to the strobo interrupt signal W.sub.3, as a strobo blocking signal. When the signal is so utilized, the transistor 30 can be rendered conductive immediately when the synchro contacts 22 are closed. Specifically, the transistor 30 is supplied with a circuit voltage in response to the closure of the synchro contacts 22, and is also supplied with a strobo blocking signal L.sub.0 of a low voltage, and is hence immediately rendered conductive to fire the thyristor 27. This causes the conduction of the extinction discharge tube 24 to block the flashlight illumination.
Referring to FIG. 2(C), there is illustrated a more rapid shutter operation which takes place with an exposure period of 1/125 second. In this instance, after the running of the first shutter blind 9 is initiated, the second shutter blind 10 will be freed from the electromagnet 6 to initiate the running before the first blind 9 comes to a stop. As a consequence, the synchro contacts 22 cannot be closed during the shutter operation. However, as the electromagnet 6 is deenergized, the back e.m.f. developed thereacross produces a strobo interrupt signal W.sub.4 shown in FIG. 2(C) to the terminal A.sub.1 at time of 1/125 second, for example. The high voltage signal W.sub.4 often erroneously renders transistor 30 conductive even though the circuit voltage is not applied thereto, thus firing thyristor 27 to discharge the capacitor 26. This causes an inconvenience that the thyristor 23 cannot be reversed biased at the next time a picture is taken. With certain cameras in which a thyristor such as thyristor 27 is used in place of the synchro contacts, the strobo interrupt signal of high voltage may cause a flashlight illumination when it is undesirable.
The described inconvenience can be avoided by removing the strobo interrupt signal as a noise when the synchro contacts 22 are not closed or when no circuit voltage is applied to the transistor 30. This also brings forth the advantage that the back e.m.f. signal W.sub.0 which occurs during the time the power switch is turned on can be completely eliminated as noise.
It will be seen from the foregoing description that the signal system of the type described must be capable of accepting signals of substantially different magnitude when the circuit voltage is applied as illustrated by FIGS. 2(A) and (B) and capable of rejecting at least high voltage signal as noise when no circuit voltage is applied as illustrated in FIG. 2(C).
With the conventional strobo circuit as illustrated in FIG. 4, a negative signal applied to the input terminal A.sub.1 from the camera will find a path including resistor R.sub.2, the gate of thyristor 27, its cathode, resistor R.sub.3, the emitter and base of transistor 30, diode 31 and resistor R.sub.5. The current flow across the terminals A.sub.1 and B.sub.1 when a circuit voltage is applied to the transistor 30 will be different from a corresponding current flow when no circuit voltage is applied to the transistor 30. Denoting the current flow across the terminals A.sub.1 -B.sub.1 when no circuit voltage is applied to the transistor 30 by I.sub..alpha. and that when circuit voltage is applied to the transistor 30 by I.sub..beta., they can be given by the following approximations: ##EQU1## where V.sub.AB represents the voltage across the terminals A.sub.1 -B.sub.1 and H.sub.FE the amplification factor of the transistor 30. When the circuit parameters are chosen so that the thyristor 27 can be turned on by a low voltage signal on the order of -2.5 V when the circuit voltage applied to the transistor 30 is available, by choosing H.sub.FE equal to 100, the resistance of resistors R.sub.2, R.sub.3 and R.sub.5 equal to 1 K.OMEGA., 100.OMEGA. and 30 K.OMEGA., respectively, the current flow will be estimated as follows: ##EQU2## and the ratio therebetween will be as follows: EQU I.sub..alpha. :I.sub..beta. =9:2
Thus, I.sub..alpha. may be as great as 4.5 times the magnitude of I.sub..beta.. This makes it difficult to remove a high voltage signal on the order of -90 V as a noise when no circuit voltage is applied to the transistor 30. Thus it will be seen that the conventional circuit described above fails to provide a required signal selection by mere choice of the circuit parameters.